1) Field of the Invention
The field of the present invention relates to methods and apparatus for multiple access communication.
2) Background
A variety of techniques are known for allowing multiple users to communicate wirelessly with one or more fixed stations (i.e., base stations) by making use of shared communication resources. Examples of multiple-access communication systems include cellular telephone networks and local wireless communication systems, such as wireless private branch exchange (PBX) networks. In such multiple-access communication systems, transmissions from different sources may be distinguished in a variety of manners, such as on the basis of different frequencies, time slots, and/or codes, for example.
As referred to herein, a communication system in which transmissions are distinguished according to the transmission frequency may be referred to as a frequency division multiple access (FDMA) communication system. A communication system in which a forward link transmission over one frequency is paired with a reverse link transmission over a different frequency may be referred to as a frequency division duplex (FDD) communication system.
A communication system in which transmissions are distinguished according to the relative timing of the transmission (i.e., by use of time slots) may be referred to as a time division multiple access (TDMA) communication system. A communication system in which a forward link transmission during one time slot (or time segment) is paired with a reverse link transmission occurring during a different time slot (or time segment) may be referred to as a time division duplex (TDD) communication system. The DECT system is an example of a well known type of TDD communication system.
A communication system in which transmissions are distinguished according to which code is used to encode the transmission may be referred to as a code division multiple access (CDMA) communication system. In a CDMA communication system, the data to be transmitted is generally encoded in some fashion, in a manner which causes the signal to be "spread" over a broader frequency range and also typically causes the signal power to decrease as the frequency bandwidth is spread. At the receiver, the signal is decoded, which causes it to be "despread" and allows the original data to be recovered. Distinct codes can be used to distinguish transmissions, thereby allowing multiple simultaneous communication, albeit over a broader frequency band and generally at a lower power level than "narrowband" FDMA or TDMA systems. Different users may thereby transmit simultaneously over the same frequency without necessarily interfering with one another.
Various "hybrid" communication systems incorporating aspects of more than one multiple access communication technique have been developed or proposed. For example, a GSM system may be viewed as a "hybrid" communication system utilizing aspects of both FDD and TDMA. In a GSM system, each base station is assigned a transmission frequency band and reception frequency band. The base station transmits to each of its mobile stations using a transmission frequency within its assigned frequency band, and the mobile stations transmit to the base station using a frequency within the base station's reception frequency band. The transmissions to the user stations are sent in assigned time slots over the base station's transmission frequency, and the transmissions from the user stations are sent in corresponding assigned time slots over the base station's reception frequency.
FIG. 3 is a diagram showing an over-the-air frame structure 301, commonly associated with a conventional GSM system. As shown in FIG. 3, a base transmission time frame 302 is defined with respect to a base station transmission frequency 311, and a mobile station transmission time frame 303 is defined with respect to a mobile station transmission frequency 312. The base station transmission frequency 311 and mobile station transmission frequency 312 are separated by a predefined frequency separation (e.g., 45 MHz). The base transmission time frame 302 comprises a number of base transmission time slots 306 of equal duration Likewise, the mobile transmission time frame 303 comprises a number of mobile transmission time slots 307 of equal duration. Both the base transmission time frame 302 and the mobile transmission time frame 303 have the same number of time slots 306, 307, such as eight time slots 306, 307 apiece.
In operation, a GSM base station transmits during the base transmission time slots 306 and receives during the mobile transmission time slots 307. The mobile transmission time frame 303 is "offset" by a predefined duration 305 (e.g., three time slots) from the base transmission time frame 302, so as to allow the mobile stations a sufficient "turn-around" switching time and information processing time, and also to allow propagation of the base-to-mobile messages to the mobile stations.
While multiple access communication may be achieved using techniques of either FDMA, TDMA or CDMA, or certain variations (e.g., FDD or TDD) or combinations thereof, problems can occur if an equipment manufacturer or operator desires to migrate from one type of multiple access communication to a different type. This problem results from the fact that equipment manufactured specifically for any one type of multiple access communication system typically cannot be used with another ye of multiple access system because of inherent differences in the nature of the communication techniques, leading to incompatibilities between the physical hardware as well as the communication protocols employed by the two communication systems. For example, a base station designed for TDD communication cannot be expected to communicate properly with an FDD handset, nor can it be expected that a TDD handset will communicate properly with a base station designed for FDD communication.
It may nevertheless be desired by equipment manufacturers or service providers to deploy or offer systems using different multiple access communication techniques or protocols, in order to serve different markets, geographical regions, or clientele, or for other reasons. However, to develop separate equipment for operation in different multiple access communication environments can substantially increase equipment design and manufacturing costs. Such a development process can also lead to the creation of different and incompatible protocols, which can require, for example, different types of backhaul service, leading to greater design expense to support the different backhaul formats and possibly duplicative base station controllers in the same local area, each servicing a different type of base station (i.e., FDD vs. TDD). Furthermore, an equipment manufacturer or service provider may desire to migrate from one type of multiple access communication and protocol to another type, without incurring substantial redesign costs.
It would therefore be advantageous to provide an apparatus and method allowing communication in more than one multiple access communication environment. It would further be advantageous to provide a method and apparatus for converting or adapting equipment from one type of multiple access communication service (e.g., TDD) to a different type (e.g., FDD).